In modern papermaking processes and machines the first part of the sheet forming operation is typically characterized by the provision of a headbox for directing a jet of papermaking slurry from the slice of the headbox and impinging it onto the upper surface of a porous wire which moves longitudinally along the production line. The wire moves over and is supported by a forming board which typically consists of a leading forming board strip followed by two or three downstream strips which are spaced from each other and spaced from the leading strip in order to permit water drainage. The slurry carries the fibers, fines and fillers onto the top of the rapidly moving wire and deposits them in a layer from which the water of the slurry is then drained. The forming board supports the wire generally in the region where the slurry impinges upon the wire.
The forming board, and particularly the first strip of the forming board, provides a support surface immediately beneath the wire to gently retain the slurry which impinges upon the wire, which at the leading strip has no fiber mat built up upon it. The leading strip aids the retention of the fiber mat on the wire. This allows an initial fiber mat to be created which further aids in the retention of fibers, fines and fillers to form a high quality sheet. As the wire progresses along the forming board and subsequent foils, drainage and doctoring of water from beneath the wire occur. It is therefore no exaggeration to say that the interaction of the fiber and fines particles in the few seconds or fractions of a second of the forming process is the very heart of papermaking.
Unfortunately, however, the forming boards of modern papermaking machines are adjustable only upon initial installation or when the machine is not operating. They are typically mounted by bolts which are accessible but do not permit adjustment of the forming board position during the dynamic operation of the machine in paper production.
Commonly, adjustments and other changes in the machine and process parameters occur as the paper mill manufactures paper to different product specifications or with different slurries. For example, the stock thickness at the slice or the slice opening may be varied or the jet angle may be modified depending on the bottom lip/upper slice geometry. Since the slurry jet which is emitted from the slice obeys conventional laws of fluid flow physics and more particularly the fundamental principles of jet trajectory, I have observed that variations in such parameters cause both the position at which the jet intercepts the wire and the longitudinal length of the jet intercept,longitudinally of the production line, to vary. The result has been that changes in the position and length of the intercept cause variations in product quality and uniformity.
For example, if the production process rate is increased, the speed of the wire must be increased and therefore the jet velocity must be increased so that the fibers of the slurry arrive on the wire at approximately the wire speed. However, increased jet velocity, which is required to deliver stock at a higher rate to the wire, causes the jet impingement points to occur further from the slice at a given predetermined angle. The longitudinal length of the intercept also varies because the angle of impingement varies and the slice opening varies too.